A camera typically comprises a photosensor, such as a CCD (charge coupled device) or CMOS (complimentary metal on silicon) photosensor, on which light from a scene imaged by the camera is focused by the camera's optics during an exposure period of the camera to acquire an image of the scene. The photosensor comprises a plurality of light sensitive pixels that are generally configured in an array of rows and columns of pixels. A pixel in the photosensor collects and registers light incident from a region of the scene imaged on the pixel by the camera optics by accumulating positive or negative electric charge provided by electron-hole pairs generated by the incident light.
The electron-hole pairs are generated in a depletion zone of a photodiode region of the pixel. A portion of a pixel occupied by its photodiode region hereinafter also referred to as a photodiode is referred to as a fill factor of the pixel. Generally, pixel sensitivity to incident light increases as its fill factor increases. To prevent cross talk between adjacent pixels during an exposure period, photodiode regions of adjacent pixels are separated by relatively deep barrier regions, referred to as channel stops, that prevent photocharge generated in one photodiode region from migrating to the photodiode or storage region of the adjacent pixel.
Which of electrons or holes are accumulated by the photosensor to accumulate photocharge that measures incident light depends on the doping structure of the photosensor. Charge provided by electron hole pairs generated by incident light may be referred to as photocharge. Electrons originating from electron hole pairs may be referred to as photoelectrons.
Photocharge generated in the photodiode region of a pixel during the exposure period is typically, temporarily stored in the pixel's photodiode region, and towards the end of the exposure period is transferred to a photocharge storage region of the pixel underlying, generally, a plurality of “gate” electrodes. Photocharge transferred to the storage region of each pixel is transferred to a read out amplifier that generates a voltage responsive to the amount of charge it receives. The voltage provides a measure of an amount of light incident on the pixel during the exposure period. Transfer of photocharge from the photodiode regions of the pixels in the photosensor to their storage regions and from the storage regions to the readout amplifier is accomplished by applying a sequence of voltages to the gate electrodes of the pixels. A set of voltages for all the pixels in the photosensor is referred to as a frame of the photosensor. The voltages in a frame may be used to display an image of the scene.
In an interline photosensor, alternate lines of pixels comprised in the photosensor may be independently controlled to collect and register light from a scene. In particular, odd and even columns of pixels may alternately be activated to receive and register light from a scene.
For example, in a gated time of flight (TOF) three dimensional (3D) camera that acquires distances to features in a scene, odd and even columns of pixels in an interline photosensor of the camera may be controlled to provide different measurements of light from the scene that are used to determine the distances. In such a camera a pulse train of short light pulses from a suitable, usually infrared (IR), light source is transmitted to illuminate the scene. Odd columns of pixels may be shuttered on, also referred to as “gated on” or “turned on”, following each of a first subset of light pulses in the pulse train for a relatively short exposure period to register light that is reflected from the light pulse by features in the scene. Even columns of pixels may be shuttered on following each light pulse in a second subset of light pulses in the pulse train for a relatively long exposure period to register light that is reflected from the light pulse by features in the scene. Light registered during the short and long exposure periods is, conventionally, often referred to as “gated” and “ungated” light respectively.
Following a last light pulse in the light pulse train, pixels in the odd and even columns of pixels in the photosensor are read to provide a frame of voltages that represent amounts of gated and ungated light registered by the photosensor for each of the features in the scene that is imaged on the pixels. Voltage in the frame responsive to gated light from a feature may be normalized to voltage in the frame responsive to ungated light from the feature and the normalized voltage used to determine distance to the feature. Distances to the features provide a range image of the scene.
Generally, intensity of light in the pulse train of transmitted light pulses is limited and amounts of light that are reflected by features in a scene from the light pulses that are available for determining distances to the features are relatively small. As the amounts of light available for determining distances decreases, accuracies with which the distances may be determined decreases.
For a TOF 3D camera using an interline photosensor for which odd columns of pixels are used to measure gated light and even pixel columns are used to acquire measurements of ungated light only about half of the available light collecting area of the photosensor is used to collect and register light for each of the measurements. Whereas the TOF 3D camera provides advantages in acquiring measurements of gated and ungated light from a same light pulse train and in a same frame of the photosensor, using only about half of the light collecting area of the photosensor for each of the measurements may degrade accuracy with which distance measurements are provided by the camera